The present invention relates to a method and a device for surface inspection for inspecting very small foreign objects on a surface of a substrate such as a semiconductor wafer or for checking micro-size flaws such as crystal defects.
In a manufacturing process of semiconductor devices, very small foreign objects are often attached to a surface of a substrate such as a wafer, and this exerts serious influence on product quality and yield. For this reason, surface inspection is performed on the surface of the substrate in the process to manufacture semiconductor devices. Semiconductor devices are now produced with increasingly higher density, and the manufacturing process is also complicated. As a result, diverse types of films are formed on the surface of the wafer.
In the surface inspection device, an inspection light is projected to the substrate surface, and reflected scattered light components caused by foreign objects are received by a detector. Thus, the foreign objects are detected. In order to attain higher detection accuracy in the surface inspection, it is necessary to maintain an S/N ratio to detect and distinguish the foreign objects. For this purpose, it is necessary to set inspection conditions, under which the reflected scattered light components from the foreign objects have a sufficiently high light amount.
A wavelength and intensity of the detection light projected to the substrate surface is related to detection sensitivity and detection accuracy. By shortening the wavelength, the detection sensitivity can be improved. By attaining sufficient intensity of the reflected scattered light components, it is possible to increase an S/N ratio in the detection and to improve detection accuracy.
Referring to FIG. 5, description will be given below on general features of a conventional type surface inspection device.
In the figure, reference numeral 1 denotes a light source unit, 2 is a projecting optical system, 3 is a photodetection unit, 4 is a rotary driving unit, 5 is a substrate such as a wafer to be inspected, and 6 is a control unit.
The rotary driving unit 4 comprises a rotary motor 7 and a rotary table 8 rotated by the rotary motor 7. The substrate 5 is fixed on the rotary table 8, and the rotary motor 7 is controlled so as to be rotated at a constant speed with a predetermined number of revolutions by a driving unit 9 based on a command from the control unit 6.
The light source unit 1 comprises a first laser emitting unit 11 and a second laser emitting unit 12 for emitting laser beams with a wavelength λ1 and a wavelength λ2 respectively. As emitters used in the first laser emitting unit 11 and the second laser emitting unit 12, a laser diode (LD) is generally used, which is easy to handle and is advantageous in terms of safety and long service life.
Laser beams emitted from the light source unit 1 are projected to the substrate 5 via the projecting optical system 2. The projecting optical system 2 comprises a first mirror 13 for guiding the laser beam with the wavelength of λ1 emitted from the first laser emitting unit 11 toward a lens unit 15, a second mirror 14 for guiding the laser beam with the wavelength of λ2 emitted from the second laser emitting unit 12 toward the lens unit 15, and a third mirror 16 and a fourth mirror 17 for projecting the laser beams from the lens unit 15 toward a point to be inspected on the substrate 5.
The first mirror 13 allows the laser beam with the wavelength λ1 from the first laser emitting unit 11 to pass and reflects laser beams with other wavelengths. The second mirror 14 reflects the laser beam with the wavelength λ2 from the second laser emitting unit 12. Then, this laser beam with the wavelength λ2 is allowed to enter the first mirror 13 so that it runs along the same optical axis as the laser beam of the wavelength λ1. The lens unit 15 adjusts the flux condition of the laser beams so that the laser beam from the first laser emitting unit 11 and the second laser emitting unit 12 are converged at the point to be inspected.
When the laser beam is projected to the substrate 5, reflected scattered light components are caused by foreign objects, flaws, etc. and the reflected scattered light components are detected by a photodetector 18. Then, data are inputted to the control unit 6 via a signal processor 19.
Depending on the size of the foreign object to be detected, either the first laser emitting unit 11 or the second laser emitting unit 12 of the light source unit 1 is selected by the control unit 6, and the laser beam is emitted. The laser beam is projected to the point to be inspected via the projecting optical system 2.
The control unit 6 rotates the substrate 5 at a constant speed by the rotary motor 7 via the driving unit 9. Then, a projecting position is shifted in a radial direction by a scanning unit (not shown), and the laser beam is controlled so that the laser beams scan spirally over the entire surface of the substrate 5.
The reflected scattered light components detected by the photodetector 18 are produced as an electrical signal. At the signal processor 19, signal processing such as amplification, removal of noise, A/D conversion, etc. is performed on the electrical signal, and then it is inputted to the control unit 6. Based on the signal from the signal processor 19, the control unit 6 detects foreign objects, flaws, defects, etc. Then, a position and a number, etc. of the foreign objects are calculated. These are recorded in a storage unit (not shown) as detection results, or inspection results are displayed on a display unit (not shown).
As the device for surface inspection, for example, those described in JP-B-8-20371 and JP-A-2000-294610 are known.
As described above, the detection sensitivity and detection accuracy in the surface inspection are influenced by intensity, i.e. a light amount, of the reflected scattered light components as detected. However, on a surface of an object to be inspected, which has light transmissivity, reflection characteristics on the surface, i.e. an important factor of the reflected scattered light components, are changed according to a thickness or a type of a film formed on the substrate surface.
For instance, when an inspection light having a certain wavelength is projected to a substrate where a same type of film is formed, reflectivity of the surface is periodically fluctuated according to the change of the thickness of the film formed on the surface. In a specific film thickness, there may be possibility that the detection accuracy to detect foreign objects or defects may be extremely decreased.
For this reason, reflectivity is extremely decreased under a certain condition depending upon the wavelength of the laser beam emitted from the light source in the surface inspection device components, i.e. the value of the wavelength of the laser beam used for the inspection, and also upon the thickness of the film formed on the substrate to be inspected. As a result, intensity of the reflected scattered light components, i.e. the value of the light amount to be detected, is extremely decreased, and this exerts influence on the accuracy of the surface inspection. For instance, when inspection is performed on substrates, which are different in film thickness even though which have the same film type, or under inspection condition where film thickness varies on the same substrate, fluctuation of reflectivity, i.e. variation in intensity and the light amount of the reflected scattered light components, occurs. This gives influences on the accuracy of the surface inspection, and the perfect inspection cannot be performed.
On the other hand, when laser beams with different wavelengths are used at the same time, extreme decrease of the intensity of the reflected scattered light components can be prevented. However, peak values of intensity of the reflected scattered light caused by the change of film thickness vary extremely in each wavelength. Dynamic range of the detection sensitivity must be set to wider range, and this increases the influence from other causes such as noise. On the other hand, the reflectivity varies according to the type of the film formed on the surface, and the type of the film also exerts influence on the detection accuracy.